In a four-cycle internal combustion engine mounted in an automobile, or the like, generally, an electronically controlled fuel injection system is employed for injecting fuel by use of electronic control. In this system, as illustrated in FIG. 9 for example, a basic fuel injection amount is determined from the intake air volume flowing in the air valve and the rotational speed of the engine. Further, a fuel injection amount suited to respective operational states of the engine is determined, by making various increase corrections according to engine operation information, such as water temperature, intake air temperature, throttle opening, and the like, or by making an air/fuel ratio correction by detecting the amount of oxygen in the exhaust pipe by using an oxygen sensor.
As a way to detect the intake air volume, an intake negative pressure sensor type (D-jetronic) intake air pressure detecting device is known. This intake air pressure detecting device is based on the principle that the engine charging efficiency is directly proportional to the intake pipe pressure. Hence, it determines the absolute pressure in the intake pipe, and then determines the intake air volume by deriving the air density taken in from the found absolute pressure and the separately measured air temperature, on the basis of the Boyle-Charles law.
In a four-cycle multiple cylinder engine mounted in an automobile, or the like, generally, a composition is adopted whereby the number of throttle valves is less than the number of cylinders, on the upstream side of the intake path, in order to adjust the intake air volume. The intake of air is then routed to the plurality of cylinders via this one or two throttle valves.
In this composition, if the degree of opening of the throttle valve is very slight, then the pressure in the intake pipe (intake pressure) is very low. For example, in some cases, it may be lower than approximately −80 kPa (gauge pressure). If, on the other hand, the load on the engine is great and the throttle valve is completely open, then the pressure in the intake pipe will rise to practically atmospheric pressure, in other words, approximately 0 kPa (gauge pressure). In engines equipped with a supercharger, it will be pressurized to approximately +101.32 kPa (gauge pressure).
Therefore, an intake pressure detecting device for detecting the absolute pressure in an intake pipe of an engine of this kind is set so as to measure a range from approximately 20 kPa (absolute pressure) to approximately 106.7 kPa (absolute pressure), which is the atmospheric pressure level under high pressure conditions, or approximately 200 kPa (absolute pressure) in order to be compatible with a supercharged engine.
An intake pressure detecting sensor constituting an intake pressure detecting device of this kind has been developed, which operates with a single DC +5V power source and also has an output voltage in the range of DC 0-5V, in order to achieve satisfactory interfacing, and which has the linear characteristics shown in FIG. 10. The conventional range of use of this intake pressure detecting sensor covers the wide region illustrated by S in FIG. 10.
In a system using the aforementioned intake pressure detecting sensor, the analogue signal of the output voltage corresponding to pressure detected by the intake pressure detecting sensor 1 is converted to a digital signal by the A/D converter 2, as illustrated by the block diagram in FIG. 11. Calculation of the intake air volume, and the like, is performed by a CPU 3 on the basis of this digital signal, and the like.
The A/D converter 2 is connected to a 5V power supply 4 forming a reference voltage, and the analogue signal of the intake pressure detecting sensor 1 is converted to a digital signal on the basis of this reference voltage. Here, if the resolution of the A/D converter 2 is 8 bits, for example, then the input voltage in the range of 0-5V is converted to a digital value in the range of 0-255.
An engine mounted in a motorcycle, snowmobile, leisure boat, or the like, will often have fewer cylinders than an engine mounted in an automobile, or the like. Further, it may be a two-cycle engine instead of a four-cycle engine. Therefore, if a conventional intake pressure detecting device of the aforementioned kind is used directly in an engine mounted in a motorcycle, snowmobile, leisure boat, or the like, then problems of the following kind arise.
Specifically, in a motorcycle, snowmobile, leisure boat, or the like, in some cases, a throttle valve is provided for each cylinder in a multiple cylinder engine, in order to emphasize accelerator response. Therefore, the pressure inside the intake pipe increases markedly. In other words, the negative pressure in the intake pipe declines markedly, compared to cases where fewer throttle valves than the number of cylinders are provided, as in the prior art. Further, in a two-cycle single-cylinder engine having a small exhaust capacity, for example, the pressure in the intake pipe may only fall to approximately −20 kPa (gauge pressure), at minimum.
Consequently, if the conventional intake pressure detecting device is used directly, then in the narrow pressure range of −20 kPa to 0 kPa (gauge pressure), the sensor output will have the narrow range indicated by L in FIG. 10 (approximately 3.2-4.0V), and approximately ⅕ of the original measurement range will actually be used. Therefore, the detectable pressure range (1 LSB) will be a relatively large value of approximately 0.51 kPa, which means that the detection sensitivity will decline and control of the fuel injection amount of the engine, and the like, becomes difficult.
Furthermore, assuming that the sensor output corresponding to the output range of −20 kPa to 0 Kpa as described above is amplified directly, then the input voltage range of the A/D converter 2 can easily be exceeded, even at a low amplification factor, and hence pressure detection becomes difficult.
If, on the other hand, the resolution of the A/D converter 2 is increased, from 8 bits to 10 bits, for example, then it becomes possible to detect pressure change up to the level of approximately 0.13 kPa. However, costs increase, software processing becomes more complicated, and the S/N ratio of the control circuit must be set to a high value.
Moreover, if used in a location of high altitude above sea level, such as in a snowmobile, it is necessary to use an intake pressure detecting sensor as an altimeter to measure atmospheric pressure. Furthermore, problems arise in that the measurement range of the intake pressure detecting sensor is narrowed due to the decline of the pressure in the intake pipe at high altitude, and the like.